This invention relates to an air bag that mitigates the impact a passenger receives at the time of collision of a car, and protects the passenger. More particularly, this invention relates to a base fabric for an improved air bag for obtaining a lightweight air bag that can be folded into a compact accommodation or stowable shape hard to unfold or revert, with repulsion, to a loosely folded state, and allows the air bag to smoothly deploy into a predetermined shape at the time of inflation.
It has become a common in the art to provide a passenger car with an air bag as a passenger protection safety device. The air bag as the passenger protection safety device is generally fitted in a module having a built-in inflator, and installed in a limited place such as a steering wheel or an instrument panel. Under such circumstances, an air bag is stowed only in a small accommodation space and is expected to be light in weight.
Reduction of the space or room to accommodate the air bag has been strongly required in recent years to increase a open space provided in the steering wheel for the sake of viewing instruments such as a speedometer panel or to increase the cabin space of the car as much as possible. To satisfy these requirements, an improved base fabric for an air bag has been required. The base fabric should have foldability for making the folded air bag more compact in size. The base fabric should be improved in the aspects of the properties so as not to deteriorate the deployment ability of the air bag, and should be more flexible.
There are two types of base fabric available for air bag. One is a base fabric provided with an elastomer coating layer such as a silicone resin to a woven fabric, and the other is an uncoated air bag base fabric not having the elastomer coating for the purpose of obtaining a light weight air bag and curtailing the coating cost.
Mechanical properties of a coated fabric are enhanced due to the presence of the coated layer provided on the base fabric. However, a base fabric for use as an uncoated air bag can not be expected to have the coated layer provided by which mechanical properties of the fabric are enhanced. Accordingly, in an uncoated air bag, deterioration in tear strength of the base fabric is observed in an accelerated heat resistance test (at 120 degree Celsius for 400 hours) which simulates an air bag equipped under an elevated temperature over a long time.
Attempts are known from Japanese Unexamined Patent Publications (Kokai) No. 5-339840, No. 6-228879, No. 10-15 60750, etc, to mitigate the deterioration in the tear strength by applying an oiling agent. Japanese Unexamined Patent Publication (Kokai) No. 5-339840 describes an air bag base fabric that is produced by allowing 0.08 wt % to 0.8 wt % of an oiling agent to be imparted to a woven fabric woven on a water jet loom by using a polyamide fiber having a yarn size of up to 450 deniers for both warp and weft and omitting scouring and has a cover factor of at least 1,700. This base fabric also prevents the deterioration in the flame proofing property. Japanese Unexamined Patent Publication (Kokai) No. 6-228879 describes a production method of a woven base fabric for an air bag that is a scoured woven fabric having a cover factor of at least 1,700 formed of a fiber having a yarn size of not greater than 450 denier, and that contains 0.6 to 5.0 wt % of an aqueous oiling agent imparted to the fabric on the basis of the fabric by means of impregnation so that the deterioration in flame proofing property of the fabric is lessened. Japanese Unexamined Patent Publication (Kokai) No. 10-60750 describes a base fabric for an air bag, woven by a Nylon 6 fiber, having a cover factor of at least 2,100, air permeability of not greater than 20 cc/cm2/sec, and containing up to 0.2% of a heat resistant lubricant for reducing the tear strength.
Attempts have been made in the base fabric for the air bag to use fine fibers for forming the woven fabric in order to further reduce the weight of the air bag and its stowability in the air bag casing.
The WO/99/22967 specification filed by the applicant of the present application describes a base fabric for an air bag to obtain an air bag having a smaller weight. In comparison with a known air bag for a driver""s seat that comprises a base fabric for an air bag using nylon 66 yarn of 350 dtex and having a thickness of 0.28 mm and Metsuke (basis of weight per unit area of fabric) of 170 g/m2, and has a bag body weight of 200 g and a folding thickness of 30 mm or more, this prior art technology is directed to reduce both weight and thickness by about 20%, preferably 30%. The base fabric for the air bag of this prior art is formed of a polyamide type synthetic fiber containing 30 to 100 ppm of a copper compound calculated as copper, and is woven by warp and weft having a single filament size of 1.0 to 3.3 dtex and a yarn size of 66 to 167 dtex. Example 1 of the specification of this prior art reference describes a 60-liter air bag for a driver""s seat using a woven fabric produced in the following way for a base fabric. Multifilament yarns of 78 dtex/35 filaments are obtained from polyhexamethylene adipamide containing 70 ppm of copper and having a formic acid relative viscosity of 60. The woven fabric is formed of the warp of a doubled yarn (sized yarns) and the weft of a doubled yarn, and is then scoured to prepare a woven fabric having a weave density of 95 ends/2.54 cm and 93 picks/2.54 cm (woven yarn size parameter: 14,820 end.dtex/2.54 cm for the warp direction, 14,508 pick.dtex/2.54 cm for the weft direction), thickness of 0.198 mm and a Metsuke (basis of fabric weight per unit area fabric) of 125/m2. The woven fabric exhibits a tensile strength retention ratio of 95% after the accelerated heat resistance test described above.
However, the air bag base fabric using the woven fabric of the polyamide fiber having such a small filament size involves the problem that the tear strength of the base fabric remarkably deteriorates in the accelerated heat resistance test. In the case of the high weave density woven fabric composed of yarns having a small filament size, another problem is encountered in that a sizing agent for weaving remains among the small size filaments without being scoured off, and accelerates combustion. It is indeed amazing that in such a woven fabric having a small filament size, an extremely small amount of a residual oil cannot improve a tear strength retention ratio, and the oiling agent must be again imparted to the small filament size woven fabric after scouring in order to pass the combustion test.
The inventor of the present invention has produced an air bag in accordance with the description of the WO99/28164 specification by using an air bag base fabric comprising a woven fabric formed of polyamide fiber used for both warp and weft each having a small filament size but by using a different weaving method and different post-treatment (scouring) of the woven fabric. The present inventor has subjected this air bag to a deployment test and observed it through a high-speed VTR. As a result, the present inventor has found that not only folding performance of the air bag but also its deployment performance depend on the weaving method and the application of scouring to the woven fabrics of the polyamide fibers having a small filament size, that is, on adhesion of the oiling agent applied during the production process of the base fabric, and that the bending property of the woven fabric that reduces repulsive unfolding of the air bag and its surface properties such as surface coarseness and the frictional property are also the factors affecting smooth deployment of the air bag. The present inventors have thus clarified that the woven fabric of the polyamide fiber having such properties can be accomplished by use of a non-sized yarn woven fabric that does not at all require desizing and scouring treatment when a polyamide fiber having a yarn size within a specific region is woven on an air jet loom.